The invention relates to a coupling sleeve for connecting to at least one pipe section fitted in an insertion end thereof, which coupling sleeve comprises an inner bush made of thermoplastic material and an outer bush, in which inner bush resistance wires are accommodated, which resistance wires can be connected to an electric power source for melting the surfaces of the coupling sleeve which face each other and those of each pipe section, in order to create a welded joint, which inner bush and outer bush in axial section have parts which mesh with each other, in order to transmit axial and/or tangential forces.
A coupling sleeve of this type for connecting two pipe sections to each other is known from EP-A-693652. This known coupling sleeve has a number of concentric casings which are connected to each other by means of meshing teeth with a block-shaped cross-section. The object of these teeth is first of all to provide better heat dissipation during the moulding of the casings in a mould, and said teeth have the additional advantage of better mutual adhesion of the casings.
Pursuant to the invention, it has been recognized that a coupling sleeve designed in this way is not suitable for use in the case of high-pressure pipes. In such cases the coupling sleeve is exposed to high tensile forces. The inner bush of the coupling sleeve welded to the ends of two pipe parts is stretched in the process. In view of the uniformity of the forms meshing with each other, the load is concentrated initially on the outermost forms meshing with each other. This causes a give-way mechanism, in which the forms give way one after the other, beginning at the outermost and continuing to the teeth situated further in.
The invention also relates to coupling sleeves which can be fixed to a pipe at the insertion end, and which bear, for example, a fixing flange at the other end.
The object of the invention is to provide a coupling sleeve which is actually suitable for use in the case of high-pressure pipes. That object is achieved by the fact that the inner bush has at least one part with a flank which faces the insertion end and in an axial plane is determined by a tangent forming an angle of less than 45xc2x0 with the axis of the inner bush.
With such a shape of the meshing forms, a more uniform transmission of forces is obtained, and excessively high tensions in the outermost meshing forms is avoided, and the meshing forms situated further in can also participate in the transmission of forces.
The angle between the tangents and the axis is preferably less than 30xc2x0. The most preferred angle is one less than 15xc2x0.
The coupling sleeve is connected to each of the pipe sections by means of a continuous circular weld. The transmission of forces between the pipe sections and the coupling sleeve occurs by way of a large weld surface, with the result that a very gradual pattern of forces occurs in the axial direction and tension concentrations are avoided.
In particular, the coupling sleeve can comprise an inner bush made of thermoplastic material and an outer bush, in which inner bush the resistance wires are accommodated.
The desired tensile strength in the direction of both axial ends of the coupling sleeve can be achieved by the fact that the meshing forms have an asymmetrical cross-section relative to a radial plane. The advantage is that the asymmetry gives the parts greater ability to participate in the transmission of axial forces.
This asymmetrical shape occurs, for example, in the case of meshing parts comprising teeth which have an asymmetrical cross-section with a relatively steeply slanting tooth flank and a relatively gently slanting tooth flank. The relatively steeply slanting tooth flank of all meshing parts with an asymmetrical cross-section faces away from the insertion end. These teeth do not contribute to the transmission of forces, and they are therefore kept as short as possible in the axial direction, which results in great steepness. The gradient of the relatively gently slanting tooth flank of the meshing forms which are situated at a relatively great distance from the insertion end of the coupling sleeve is smaller than the gradient of the relatively gently slanting tooth flank of the meshing parts situated a relatively short distance from the insertion end, which produces a transmission of forces distributed uniformly over the coupling sleeve.
The size of the gradients of the relatively gently slanting tooth flanks of the meshing forms decreases monotonically in the direction facing away from the insertion end.
Moreover, the coupling sleeve can be shut off fluently at each end by means of conically tapering parts.
The asymmetrical meshing forms according to the invention can be designed in various variants.
According to a first variant, the inner bush and the outer bush have separate meshing teeth running in the circumferential direction. Such teeth each form an annular thickening relative to the inner bush and the outer bush.
According to a further variant, the inner bush and the outer bush of the coupling sleeve have helically meshing tooth systems. Such a helical shape is suitable for relatively short pipe sections which can be screwed into the sleeve. The tooth system is asymmetrical, but must also have a continuous cross-section, in order to permit the screwing movement.
The coupling sleeve can be designed in various ways, depending on the potential applications. If the coupling sleeve can be pushed onto each pipe section in the axial direction, the coupling sleeve can comprise a bush which is in one piece in the circumferential direction, and in which the resistance wires are accommodated, running in the circumferential direction.
In an alternative embodiment, the coupling sleeve can comprise mutually connecting shells, in which the resistance wires are accommodated, running in the axial direction.
The outer bush is fibre-reinforced for bearing axial and/or tangential forces. The outer bush preferably comprises fibres with a tensile modulus  greater than 50 GPa, such as glass fibres, carbon fibres or aramid fibres. The outer bush also preferably comprises a thermosetting material such as polyester, vinyl ester or epoxy resin.
The coupling sleeve according to the invention can be used in various ways. First of all, the coupling sleeve is suitable for connecting two pipe sections to each other. In that case the coupling sleeve can be symmetrical relative to a radial plane of symmetry, and the meshing forms on one side of the plane of symmetry are a mirror-symmetrical shape relative to the meshing forms on the other side of the plane of symmetry.
According to a variant, the inner bush has two external forms, each tapering conically towards an axial end, and the outer bush has two correspondingly shaped internal forms, which outer bush has an essentially constant thickness.
Coupling sleeves comprising an inner bush with electric resistance wires are known. Such inner bushes are of standard dimensions and, in view of the great forces occurring in high-pressure pipes, could sometimes produce an inadequate weld. In order to be able to use such standard components in the case of high-pressure pipes in any case, the coupling sleeve can have at least two separate inner bushes situated next to each other in the axial direction and accommodated in the outer bush.
According to a second variant, the coupling sleeve has one part which is provided with an insertion end for a pipe section, and also has an auxiliary end to which a further element can be fixed. The auxiliary end can be provided with a fixing flange, by means of which the coupling sleeve can be connected to, for example, a tank.
The invention also relates to a sleeve joint between two pipe sections of a high-pressure pipe, which pipe sections each comprise a fluid-tight inner casing made of a thermoplastic material, and also a reinforced outer casing, the inner casings being connected to each other in a fluid-tight manner, and the outer casings being connected to each other by means of a coupling sleeve, as described above.
The inner casings are welded to each other directly at their end faces which face each other.
The sleeve joint can also be designed in such a way that at the ends of the pipe sections facing each other the inner casings are externally unattached. In that case the coupling sleeve has an inner bush with a central part of relatively large diameter, which central part of the inner bush is welded to the external surface of the inner casings.
The outer casings can comprise fibres with a modulus  greater than 50 GPa, such as glass fibres, carbon fibres or aramid fibres, embedded in a thermoplastic matrix.
Furthermore, the outer casings can comprise an outside layer of thermoplastic material.
The welded joint between the coupling sleeve and the pipe sections is best achieved if the inner casings and the outer casings and also the inner bush of the coupling sleeve comprise the same thermoplastic material, such as a polyolefin.
A further coupling sleeve is known from WO-A-9512086. This coupling sleeve is particularly suitable for high-pressure pipes, for example suitable for pressures up to 100 bar. The joint in the inner casings of the pipe sections is obtained by welding, and is consequently suitable for ensuring the desired tightness. An example which can be mentioned is pipes for transporting gas and crude oil.
In the case of this known sleeve joint, a coupling sleeve is placed on the ends of the outer casings facing each other. By means of the conical fixing rings, the coupling sleeve is clamped on the outer surface of the outer casing, the coupling sleeve and the pipe sections being deformed locally in order to provide the necessary axial retaining power.
This known sleeve joint has various disadvantages. First of all, the local deformation of the wall of the pipe sections can give rise to tension concentrations, which can lead to giving way, particularly at high pressures.
A further disadvantage is that several loose components have to be used for the joint, which components have to be fitted on the pipe sections beforehand.